The great nervous centres are formed from the cells of the epiblast, which, in the earliest days of the embryo, form a longitudinal furrow, which sinks into the cells of the mesoblast. By the rapid growth of the latter the depressed part is cut off from the rest of the epiblast, and forms the rudiment of the spinal cord and brain. In looking for special conducting tissue in animals possessing the most simple structure, we find cells which would seem to possess certainly a twofold, and possibly a threefold function, - one of which is conduction. In the so-called "neuro-muscular" cells of the hydra, processes are described as passing off from them, and uniting beneath the ectoderm with other fibre-like processes, which are evidently contractile. Here we find for the first time a portion of protoplasm specially devoted to acting as a conductor of impulses, and attached by the one end to a contractile fibre, and by the other to a surface (sensory) cell. The intimate relation between the development of nerve and muscle fibres is thus established, and we have the first indication of a nerve mechanism, viz., a cell capable of receiving stimulations, and a fibre capable of transmitting the resulting impulses. As further differentiation proceeds, each of these parts becomes more distinct from the other, and ultimately the adult nerve tissue is found to be made up of nerve fibres, and special cells, forming nerve endings.

* A further account of the Histology of these tissues will be found in the chapters specially devoted to these subjects.

Epithelial cells, some of which are filled with mucus (d), forming gobletlike cells.

Fig. 15. Epithelial cells, some of which are filled with mucus (d), forming gobletlike cells. (Cadtat).

Neuro muscular cells of hydra, m. Contractile fibres. (Kleinenberg).

Fig. 16. Neuro-muscular cells of hydra, m. Contractile fibres. (Kleinenberg).

S. Sensory receiving organ with attached afferent nerve fibre.

Fig. 17. S. Sensory receiving organ-with attached afferent nerve fibre.

G. Central organs - ganglion cells.

M. Peripheral organ and efferent nerve.

Three medullated nerve fibres, the medullary sheath of which is stained dark with osmic acid.

Fig. 18. Three medullated nerve fibres, the medullary sheath of which is stained dark with osmic acid. N, Nodes of Ranvier.

Two non-medullated nerve fibres, with nuclei in the primitive sheath.

The fibres act as lines of communication between ganglion cells; they connect together the numerous cells in the various parts of the brain and spinal cord, or pass between those of the central nervous organs and ganglia distributed throughout the body, which might be called the peripheral nerve organs.

The simplest idea, then, of a special nerve apparatus is a fibre connecting two cells. The peripheral cell may be a receiving organ (Fig. 17, s), from which, when stimulated, impulses are transmitted along the fibre to the central nerve cell, where they give rise to certain impressions, and so we have a sensory nerve apparatus. Or the central nerve cell may be the receiving agent, getting stimuli from its central neighbors, and transmitting impulses to a peripheral nerve terminal, by which they are handed over to a muscle (m) or gland, and thus we have a simple motor or secretory apparatus. Where the effect of a stimulus can be definitely traced from one nerve cell to another, and from thence by a second fibre to a third cell, the impulse is said to be reflected by the second cell to the third. And there we have what is called a reflex act.

Multipolar cells from the anterior gray column of the spinal cord of the dog fish.

Fig. 19. Multipolar cells from the anterior gray column of the spinal cord of the dog fish (a) lying in a texture of fibrils; (b) prolongation from cells: (c) nerve fibres cut across. (Cadiat).

The essential part of a nerve fibre is a kind of protoplasmic band, in which the finest fibrilla or thread-like marking can be made out with the aid of reagents and a powerful microscope. This is called the axis cylinder. In some nerve fibres (mostly in the brain and spinal cord) the axis cylinder is naked, and even a single fibril may so pass from one cell to another in the brain matter. In other parts the axis cylinder is generally covered by a thin membrane, called the primitive sheath, or with a soft, oillike substance, called the medullary sheath, or, as is commonly the case in most peripheral nerves, by both. The primitive sheath encloses the medullary sheath, which surrounds the axis cylinder.

These fibres are made of peculiarly modified cells, which are, however, so elongated as not to be very easily recognized as such in adult tissue.

Ganglion cells of frog, showing straight and spiral fibres.

Fig. 20. Ganglion cells of frog, showing straight and spiral fibres. {After Beale and Arnold).

Cells from the sympathetic ganglion of a cat. The protoplasm is retracted here and there from the cell wall.

Fig. 21. Cells from the sympathetic ganglion of a cat. The protoplasm is retracted here and there from the cell wall.

The nerve or ganglion cells vary extremely in general form and size. The commonest in the nerve centres are large bodies with a clear, well-defined, vesicular, single nucleus, and distinct nucleolus; they have two or more processes, which are connected by nerve fibres to other cells, and to the axis cylinder of nerves.

The peripheral nerve cells are generally much modified, and often small compared with those in the centres. Besides the cells in the sporadic ganglia, which are large rounded corpuscles with but few processes, there are many other bodies connected with the peripheral nerves which cannot be called ganglion corpuscles. They are nevertheless nerve cells.